判断滚动条是否到达页面的尾部

//取窗口滚动条高度   
function getScrollTop()   
{   
    var scrollTop=0;    
    if(document.documentElement&&document.documentElement.scrollTop)    
    {   
        scrollTop=document.documentElement.scrollTop;   
    }   
    else if(document.body)    
    {   
        scrollTop=document.body.scrollTop;   
    }   
    return scrollTop;    
}   
  
  
 //取窗口可视范围的高度   
function getClientHeight()   
{   
    var clientHeight=0;    
    if(document.body.clientHeight&&document.documentElement.clientHeight)    
    {   
        var clientHeight = (document.body.clientHeight<document.documentElement.clientHeight)?document.body.clientHeight:document.documentElement.clientHeight;           
    }   
    else   
    {   
        var clientHeight = (document.body.clientHeight>document.documentElement.clientHeight)?document.body.clientHeight:document.documentElement.clientHeight;       
    }   
    return clientHeight;    
}   
  
 
 //取文档内容实际高度   
function getScrollHeight()   
{   
    return Math.max(document.body.scrollHeight,document.documentElement.scrollHeight);    
}   
        function test(){   
    if (getScrollTop()+getClientHeight()==getScrollHeight()){    
        alert("到达底部");    
    }else{    
        alert("没有到达底部");    
    }   
        }